Since he was a young man, René Descartes wanted to discover men and the nature of the Universe. Descartes was studying philosophy to find a knowledge that was inside ourselves when he realized that he wouldn't reach his objective at the university. So he decided to travel through Europe, searching for this knowledge.

On this epic journey, Descartes started to ask himself: "how can anyone know which philosophical method a philosopher should use to solve a philosophical problem? How can anyone be sure that what they feel is real?" He noticed that dreams were very close to reality. After all, what is the difference between dream and reality?

The French philosopher tried to doubt everything. He discovered that he should doubt everything except his thoughts' reality. He said that philosophers should apply mathematical methods when they are solving a philosophical problem (break down a problem into a lot of simple problems). Reason is the most important thing in the human being's existence.

In the end, René Descartes formulated the answer to his questions - and one of the most famous sentences of history: "Cogito, ergo sum", or "I think, therefore I am".